The braincase of <em>Bissektipelta archibaldi</em> — new insights into endocranial osteology, vasculature, and paleoneurobiology of ankylosaurian dinosaurs

Abstract

We describe in detail three braincases of the ankylosaur Bissektipelta archibaldi from the Late Cretaceous (Turonian) of Uzbekistan with the aid of computed tomography, segmentation, and 3D modeling. Bissektipelta archibaldi is confirmed as a valid taxon and attributed to Ankylosaurinae based on the results of a phylogenetic analysis. The topographic relationships between the elements forming the braincase are determined using a newly referred specimen with preserved sutures, which is an exceedingly rare condition for ankylosaurs. The mesethmoid appears to be a separate ossification in the newly referred specimen ZIN PH 281/16. We revise and discuss features of the neurocranial osteology in Ankylosauria and propose new diagnostic characters for a number of its subclades. We present a 3D model of the braincase vasculature of Bissektipelta and comment on vascular patterns of armored dinosaurs. A complex vascular network piercing the skull roof and the wall of the braincase is reported for ankylosaurs for the first time. We imply the presence of a lepidosaur-like dorsal head vein and the venous parietal sinus in the adductor cavity of Bissektipelta. We suggest that the presence of the dorsal head vein in dinosaurs is a plesiomorphic diapsid trait, and extant archosaur groups independently lost the vessel. A study of two complete endocranial casts of Bissektipelta allowed us to compare endocranial anatomy within Ankylosauria and infer an extremely developed sense of smell, a keen sense of hearing at lower frequencies (100–3000 Hz), and the presence of physiological mechanisms for precise temperature control of neurosensory tissues at least in derived ankylosaurids.

Neuroanatomy of the nodosaurid Struthiosaurus austriacus (Dinosauria: Thyreophora) supports potential ecological differentiations within Ankylosauria

“…Although not completely preserved, the endocranial cavity suggests great angles of both the cerebral and the pontine flexure in IPUW 2349/6, matching the condition in other ankylosaurs and stegosaurs 11 – 13 , 32 . The endocast shows a slight dural peak (Fig.…”

“…2 E,F; vcm). Additionally, at least two large canals, tentatively assigned to the transverso-occipital vein 13 , traverse each paroccipital process and the prootic mainly anteroposteriorly; without any obvious connection to another endocranial structure (Fig. 2 A,C,E).…”

“…2 A,C,E). More delicate and complex networks of canals are present further anterodorsally and may belong to the posterior branching plexus 13 . Possibly, the expanding mediolateral width of the anterodorsal endocast marks the otherwise barely delineated cerebral hemispheres.…”

“…The presence of widely distributed vascular canals in the holotypic neurocranium of Struthiosaurus austriacus adds to the diversity of patterns within Thyreophora 6 , 13 , 14 , 35 . Conversely to Bissektipelta archibaldi 13 , these canals are not obviously interconnected with each other or the cavity that once contained the brain, and are located closer to the endosseous labyrinth in S. austriacus.…”

“…Whereas complete braincase material among early-diverging thyreophorans is only known from Scelidosaurus harrisoni 10 , neurocranial material of stegosaurs 11 , 12 , and ankylosaurs (e.g. 13 – 15 ) is more common. The heavily armored skull roofs of the latter likely improved their preservation potential.…”